Optical sensor for detecting liquid medium

ABSTRACT

An optical sensor comprises a probe (10) and an annular optical element (13) coupled optically to a pair of optical fibres (18). Light transmitted down one fibre is reflected around the element (13) and back up the other fibre to a photodetector. The level of light received by the photodetector is dependent upon the amount of light lost from the element (13) as a result of characteristics of the surrounding medium. The presence or absence of a surrounding liquid medium may thus be sensed and a motor (34) operated to suck liquid up through the probe, when the liquid is present.

Sensors incorporating light guides have been proposed in applicationswhere information may be obtained optically from a remote position. Ingeneral, light from a source is transmitted by total internal reflectionalong a light guide to a position from where information is required, adlight modulated in some way to carry the information is transmitted backalong the same or another light guide to a photodetector. One kind ofsensor is used as an optical dip-stick. It consists of a glass orplastics rod which has a bevelled tip, or of a bevelled tip, such as aprism, at the end(s) of one of more light guides. Light transmitted downone side of the rod, or down a light guide, is reflected twice at thebevelled tip and returns up the other side of the rod, or up the otherlight guide, when the tip is dry. When the tip is wet and the angle ofincidence is less than the critical angle, light is lost and thereturning optical signal is greatly reduced. A variant of this sensorconsists of a sinuous flexible optical fibre along which light istransmitted to and from an end of the sensor, light being lost from thecurved portions of the optical fibre when the fibre is wetted.

Such sensors have been used in conjunction with liquid level controlsystems, whereby, for example, a pump relay is operated in dependenceupon whether the intensity of light received by the photodetector isabove or below a predetermined threshold.

The known sensors suffer from a number of problems, such as thedifficulty of manufacturing the sensors with an accurately reproducibleresponse at a precise sensing location, the need for repeatedrecalibration owing to fouling of the surface of the light guide bydeposits or contact with foreign matter, drop retention at the sensinglocation leading to light loss when the sensor is otherwise dry,excessive sensitivity in that there is almost complete light loss whenonly a small part of the sensing location is wetted, and complete lightloss when the sensing region is wetted so that the wetted state isindistinguishable from a break in the light guide.

In accordance with the present invention, an optical sensor comprises anelongate probe; a light guide extending from a light source, along theprobe towards a tip of the probe and back along the probe to aphotodetector, the light guide including, adjacent to the tip, a rigidelement, which has a radially outer substantially cylindrical surfaceproviding substantially total internal reflection of light passingcircumferentially around the probe within the element or loss of lightdepending on the optical properties of a surrounding medium, and whichis mounted in a substantially cylindrical portion of the probe so thatthe outer surface of the element is substantially flush with the outersurface of that portion of the probe.

Preferably the element is substantially annular and is closely receivedin a groove in the probe.

The use at the sensing location of the rigid element, the lighttransmitting and reflecting surfaces of which will be optically smooth,with the element recessed into the probe with only its radially outersurface exposed, overcomes many of the disadvantages of previoussensors. The element, which will normally be a plastics moulding, willbe preformed with the necessary precision and with the appropriategeometry for the particular application. Droplet retention may bediscouraged by coating the adjacent part of the outer surface of theprobe with a hydrophobic material, such as polyurethane or silicone.

Simple assembly may be provided by mounting the element between twoparts of the probe which are connected by a spigot and socket coupling,the element surrounding the spigot. Light may then be transmitted intoand out of the element by optical fibres, which preferably extend downthe probe, protected in their own duct or ducts in the probe, with endsof the optical fibres abutting optical coupling surfaces of the elementfacing axially away from the probe tip. The surface of the elementfacing the probe tip may then be provided with indented chamferedreflection surfaces in alignment with the coupling surfaces to reflectlight from one optical fibre, circumferentially around the element and,to reflect light passing circumferentially around the element into theother optical fibre. The optical coupling surfaces may be provided bythe end walls of blind sockets, which are formed in the element andwhich receive the ends of the optical fibres. If the optical fibres andcoupling surfaces are closely angularly spaced, the light may betransmitted circumferentially around within the element through an angleof the order of 300° or more.

In use light will be transmitted around within the element withpotential total internal reflection at nodes on the radially outersurface of the element. The geometry may be such that an integral numberof theoretical reflection nodes will exist around the radially outersurface which is exposed to the surrounding medium. One can hypothesisethat the light loss at each node when it is wetted or otherwisecontacted with an appropriate medium would be a fraction, e.g. 50%, ofthe light incident at that node. Light loss at a single node, resultingfor example from some perturbation in the surrounding medium, will notthen reduce the transmitted light by more than 50%. However, if thesurrounding medium is such that light is lost from the element at allthe nodes, of which there may be between three and six, the remaininglight which is transmitted out of the element will approach zerointensity, but still be finite. Appropriate thresholds can then be setto determine whether the light loss corresponds to a significant changein the surrounding medium, and still distinguish from the case in whichall light is lost owing to a break in the light guide.

However this hypothesis depends on the light rays being effectivelycollimated, which they will not normally be. In practice they aredivergent when entering the element so that only the central rays willbe reflected at the nodes, the remaining rays being reflected at otherthan the nodes and, becoming more diffuse, and possibly even beingreflected at the radially inner surface of the element. We have foundthat an excellent response, i.e. not too great a sensitivity, can beobtained by ignoring the central rays and effectively using the diffuselight. This can be achieved by providing areas of reflectivity on theradially outer surface of the element at the nodes so that the centralrays are always internally reflected at these positions, irrespective ofthe surrounding medium. It will then be the loss of the diffuse lightinternally incident on the radially outer surface of the element betweenthe areas of reflectivity, in dependence on the wetness or otherrelevent optical characteristics of the surrounding medium, which willdetermine the intensity of light transmitted along the light guide outof the element. Because there is negligible loss of light from thecentral rays, this intensity will always be finite and appropriatethresholds can be set to determine whether the light loss corresponds toa significant change in the surrounding medium, and-still distinguishfrom the case in which all light is lost owing to a break in the lightguide. The areas of reflectivity may extend, in aggregate, around aboutone third of that part of the circumference of the element about whichthe light is transmitted. For some reason this arrangement seems not tobe ultrasensitive to contact with wet tissue.

The sensor has a wide variety of applications but in most cases, thedetector will be a photoelectric transducer, such as a photodiode, whichmay be connected into a suitable electronic circuit to produce anelectrical signal corresponding to the intensity of the optical signaldetected. The electrical signal may be fed to a display, or to recordingapparatus, or as a control signal to a pump, valve or motor associatedwith the liquid or medium being sensed, usually via a discriminationcircuit which responds to whether the signal level is above or below acertain threshold. To avoid spurious response to transient ambientlight, the light transmitted along the light guide is preferably pulsed,the circuit connected to the photodetector being responsive only to thepulse frequency.

A typical use for the sensor in a non medical field is as a levelsensor, bubble detector, colour sensor or interface detector in mixturesof gas, liquid and/or solid.

Two very important uses for these sensors in reservoirs for blood orliquids for infusion, and (b 2) in association with a liquid sucker,such as catheter for removal of blood or other body fluids in vascularand cardiac surgery, or a dentist's mouth piece for removing saliva froma patient's mouth. Conventionally, such suckers operate continuously andif they are to have sufficient capacity to remove all liquid as itaccumulates, it is inevitable that they will continually aspirate airand liquid This is irritatingly noisy and, if the liquid is blood, theresulting shear stresses in the blood are a major cause of blood traumaand the formation of gas microbubbles and fat globules. These problemscan be overcome if the liquid sucker has a tubular body providing theprobe of a sensor in accordance with the present invention. The signalreceived by the detector may then be used automatically to switch on apump or other source of suction to the sucker when the radially outersurface of the element is wetted by the liquid, and to switch off thesource of suction when the liquid level has dropped below the element

By appropriately positioning the suction tube relatively to a bodycavity, a constant level of fluid may be maintained, for example in thepericardium during topical hypothermic myocardial protection.

By using light of a particular wavelength, information about the stateof liquid adjacent to the element may be obtained For example, if thelight transmitted along the light guide is red, conveniently supplied bya red light emitting diode, and the liquid is blood, the loss of lightfrom the element and hence the optical signal received by the detector,varies with the colour of the blood. The variation in the colour iscorrelated with the oxygen saturation of the blood. Such a sensor mayprovide a valuable clincial guide to the performance of an artificiallung during open heart surgery.

A further application of the sensor is for sensing the pH of blood orother liquid, where the outer surface of the element is coated withimmobilised material which changes colour with the pH of the medium withwhich it is in contact. The change in colour of the immobilised materialwill produce a corresponding change in the level of the detected signal.Similarly the outer surface of the element may be coated with amaterial, such as an antibody, which reacts with the surrounding mediumthereby changing the thickness of the coating and hence the light lossthrough the coating.

An example of a liquid sucker, constructed in accordance with thepresent invention, is illustrated in the accompanying drawings, inwhich:

FIG. 1 is a partially diagrammatic side elevation of the sucker;

FIG. 2 is an enlarged side elevation of the end of the sucker;

FIG. 3 is a vertical section through the end of the sucker taken on theline III--III in FIG. 5;

FIG. 4 is a section taken on line IV--IV in FIG. 3;

FIG. 5 is a section taken on the line V--V in FIG. 3;

FIG. 6 is a section taken on the line VI--VI

in FIG. 3;

FIG. 7 is a section taken on the line VII--VII in FIG. 3;

FIG. 8 is a section taken on the line VIII--VIII in FIG. 6 of an opticalelement of the sucker; and,

FIG. 9 is a plan view of the optical element showing its manner ofoperation.

The sucker is formed by an elongate probe 10 formed of a substantiallyrigid bilumina plastics tube 11, to the end of which is fitted a headcomprising a plastics connector element 12, an optical element 13, and aplastics tip element 14. The peripheral, substantially cylindrical,surfaces of the elements 12 and 13, and of the elements 13 and 14, areflush with one another. A central suction lumen 15 of the tube 11continues through the elements 12, 13 and 14 and opens through fourinlets 16 between cruciform vanes 17 at the end of the tip element 14.The tube 11 also has at one side a second smaller lumen 18 providingprotection for a pair of optical fibres 19.

The connector element 12 has a spigot 20 which is a tight fit in a lowerend of the lumen 15 to couple the tube to the element 12. The element 12also has at its other end a second spigot 21 which passes closelythrough a central aperture in the optical element 13 and into a socketin the upper end of the tip element 14 coaxial with the continuation ofthe lumen 15. The element 13 is therefore effectively mounted in anannular groove around the spigot 21. The three elements are securelyfixed together by ultrasonic welding at mutually contacting lips 22, 23and 24. Proper mutual angular orientation between the elements 12 and 13is provided by a projection 25, which depends eccentrically from theelement 13 and is received in a complementary recess in the upper faceof the element 14. In this mutual angular orientation, a raised boss 26positioned eccentrically on the upper face of the optical element 13 isin alignment with the secondary lumen 18 in the tube 11. As shown inFIG. 6 and 9, the boss 26 is provided with two blind sockets 27 each ofwhich receives as a close fit the lower end of a respective one of thetwo optical fibres 19. End walls 28 of the sockets provide opticalcoupling surfaces which are abutted by the ends of the optical fibres sothat light may be transmitted between the optical element 13 and thefibres. Immediately opposite the sockets 27, the optical element 13 isprovided on its under surface with a V-shaped recess forming indentedchamfered reflection surfaces 29.

The optical element 13 is made of hard optical quality translucentacrylic material and its optical surfaces, namely the end walls 28 ofthe sockets, its peripheral wall 30 and the reflection surfaces 29 areoptically polished. The arrangement is such that when light istransmitted down one of the optical fibres 19, it passes through the endwall 28 of its respective socket into the element 13, and is incident onthe respective reflection surface 29 in a direction substantiallyparallel to the length of the probe. The light is reflected off thesurface 29 and passes around the element 13, as a result of totalinternal reflection at the peripheral wall 30. As shown in FIG. 9, thegeometry is arranged so that the central rays 31 will be reflected atthree nodes 32 before being incident on the other reflection surface 29,and hence being reflected back into, and transmitted along the otheroptical fibre 19. Narrow axially extending reflective strips 33 areprinted on to the peripheral surface 30 at positions corresponding tothe nodes 32. As explained above, this is to ensure total internalreflection of the central rays at all time. The diffuse rays whichimpinge on the surface 30 between the nodes 32 will be total internallyreflected to an extent depending upon the medium surrounding and incontact with the walls 30. This will affect the level of lighttransmitted back along the second optical fibre 19 and, as explained, isa measure of a characteristic of the surrounding medium.

The simple sucker illustrated is primarily intended to distinguishbetween the presence or absence at the lower end of the probe of aliquid which is to be sucked through the lumen 15 by a pump 4. Assuggested in FIG. 1, the optical fibres 19 may be connected to a controlbox 35, containing a light source, such as an LED, which transmits lightdown the first optical fibre 19, and a photodetector, which receiveslight transmitted back through the other optical fibre 19. The controlunit 35 will be programmed to recognise whether the received lightexceeds a predetermined threshold level, indicating an absence of liquidmedium surrounding and in contact with the surface 30. It will thenprovide a signal ensuring that the pump 34 is not operatedunnecessarily. If, subsequently, the surface 30 is immersed in liquid,more of the diffuse light will be reflected through, rather thaninternally reflected at, the surface 30 and the unit 35 will recognisethe falling of the level of received light below the threshold limit,and will provide a signal causing the pump 34 to be operated, so thatthe liquid will be sucked up through the lumen 15.

We claim:
 1. An optical sensor comprising an elongate probe; a lightguide extending from a light source, along the probe towards a tip ofthe probe and back along the probe to a photodetector, the light guideincluding, adjacent to the tip, a rigid element, which has a radiallyouter substantially cylindrical surface providing substantially totalinternal reflection of light passing circumferentially around the probewithin the element, or loss of light, depending on the opticalproperties of a surrounding medium, and which is mounted in asubstantially cylindrical portion of the probe so that the outer surfaceof the element is substantially flush with the outer surface of thatportion of the probe.
 2. A sensor according to claim 1, in which theelement is substantially annular and is closely received in a groove inthe probe.
 3. A sensor according to claim 2, in which the element ismounted between two parts of the probe which are connected by a spigotand socket coupling, the element surrounding the spigot.
 4. A sensoraccording to claim 1, in which the light guide includes optical fibresfor transmitting light into and out of the element, ends of the fibresabutting optical coupling surfaces of the element facing axially awayfrom the probe tip.
 5. A sensor according to claim 4, in which theoptical fibres extend down the probe protected in their own duct orducts within the probe.
 6. A sensor according to claim 4, in which thesurface of the element facing the probe tip is provided with indentedchamfered reflection surfaces in alignment with the coupling surfaces toreflect light from one optical fibre, circumferentially around theelement, and to reflect light passing circumferentially around theelement into the other optical fibre.
 7. A sensor according to claim 1,in which the geometry of the element is such that, in use, light will betransmitted around within the element with potential total internalreflection of central rays at an integral number of nodes on theradially outer surface of the element.
 8. A sensor according to claim 6,in which areas of reflectivity are provided on the radially outersurface of the element at the nodes so that the central rays are alwaysinternally reflected at these positions.
 9. A liquid sucker having atubular body providing the probe of a sensor according to claim 1, andmeans for processing the signal received by the detector automaticallyto switch on a pump or other source of suction to the sucker when theradially outer surface of the element is wetted by a liquid, and toswitch off the source when the outer surface is not wetted.
 10. A sensoraccording to claim 2, in which the light guide includes optical fibresfor transmitting light into and out of the element, ends of the fibresabutting optical coupling surfaces of the element facing axially awayfrom the probe tip.
 11. A sensor according to claim 3, in which thelight guide includes optical fibre for transmitting light into and outof the element, ends of the fibres abutting optical coupling surfaces ofthe element facing axially away from the probe tip.
 12. A sensoraccording to claim 11, in which the optical fibres extend down the probeprotected in their own duct or ducts within the probe.
 13. A sensoraccording to claim 10, in which the optical fibres extend down the probeprotected in their own duct or ducts within the probe.
 14. A sensoraccording to claim 10, in which the surface of the element facing theprobe tip is provided with indented chamfered reflection surfaces inalignment with the coupling surfaces to reflect light from one opticalfibre, circumferentially around the element, and to reflect lightpassing circumferentially around the element into the other opticalfibre.
 15. A sensor according to claim 11, in which the surface of theelement facing the probe tip is provided with indented chamferedreflection surfaces in alignment with the coupling surfaces to reflectlight from one optical fibre, circumferentially around the element, andto reflect light passing circumferentially around the element into theother optical fibre.
 16. A sensor according to claim 12, in which thesurface of the element facing the probe tip is provided with indentedchamfered reflection surfaces in alignment with the coupling surfaces toreflect light from one optical fibre, circumferentially around theelement, and to reflect light passing circumferentially around theelement into the other optical fibre.
 17. A sensor according to claim13, in which the surface of the element facing the probe tip is providedwith indented chamfered reflection surfaces in alignment with thecoupling surfaces to reflect light from one optical fibre,circumferentially around the element, and to reflect light passingcircumferentially around the element into the other optical fibre.
 18. Asensor according to claim 2, in which the geometry of the element issuch that, in use, light will be transmitted around the element withpotential total internal reflection of central rays at an integralnumber of nodes on the radially outer surface of the element.
 19. Asensor according to claim 3, in which the geometry of the element issuch that, in use, light will be transmitted around within the elementwith potential total internal reflection of central rays at an integralnumber of nodes on the radially outer surface of the element.
 20. Asensor according to claim 4, in which the geometry of the element issuch that, in use, light will be transmitted around within the elementwith potential total internal refection of central rays at an integralnumber of nodes on the radially outer surface of the element.
 21. Asensor according to claim 5, in which the geometry of the element issuch that, in use, light will be transmitted around within the elementwith potential total internal reflection of central rays at an integralnumber of nodes on the radially outer surface of the element.
 22. Asensor according to claim 6, in which the geometry of the element issuch that, in use, light will be transmitted around within the elementwith potential total internal reflection of central rays at an integralnumber of nodes on the radially outer surface of the element.
 23. Aliquid sucker having a tubular body providing the probe of a sensoraccording to claim 2, and means for processing the signal received bythe detector automatically to switch on a pump or other source ofsuction to the sucker when the radially outer surface of the element iswetted by a liquid, and to switch off the source when the outer surfaceis not wetted.
 24. A liquid sucker having a tubular body providing theprobe of a sensor according to claim 3, and means for processing thesignal received by the detector automatically to switch on a pump orother source of suction to the sucker when the radially outer surface ofthe element is wetted by a liquid, and to switch off the source when theouter surface is not wetted.
 25. A liquid sucker having a tubular bodyproviding the probe of a sensor according to claim 4, and means forprocessing the signal received by the detector automatically to switchon a pump or other source of suction to the sucker when the radiallyouter surface of the element is wetted by a liquid, and to switch offthe source when the outer surface is not wetted.
 26. A liquid suckerhaving a tubular body providing the probe of a sensor according to claim15, and means for processing the signal received by the detectorautomatically to switch on a pump or other source of suction to thesucker when the radially outer surface of the element is wetted by aliquid, and to switch off the source when the outer surface is notwetted.
 27. A liquid sucker having a tubular body providing the probe ofa sensor according to claim 6, and means for processing the signalreceived by the detector automatically to switch on a pump or othersource of suction to the sucker when the radially outer surface of theelement is wetted by a liquid, and to switch off the source when theouter surface is not wetted.
 28. A liquid sucker having a tubular bodyproviding the probe of a sensor according to claim 7, and means forprocessing the signal received by the detector automatically to stitchon a pump or other source of suction to the sucker when the radiallyouter surface of the element is wetted by a liquid, and to switch offthe source when the outer surface is not wetted.
 29. A liquid suckerhaving a tubular body providing the probe of a sensor according to claim8, and means for processing the signal received by the detectorautomatically to switch on a pump or other source of suction to thesucker when the radially outer surface of the element is wetted by aliquid, and to switch off the source when the outer surface is notwetted.